1. Field of the Invention
The present invention relates to a lithographic plate which necessitates no dampening water for the printing.
2. Prior Art
Many lithographic plates which necessitate no dampening water for the printing (hereinafter referred to as "waterless lithographic plate") have been already invented. They are described in, for example, U.S. Pat. Nos. 3,677,178, 3,511,178, 3,865,588, 3,894,873 and 4,342,820, and British Pat. No. 1,419,643. In particular, the photosensitive layers of positive working lithographic plates are usually classified into two types, i.e. one type is photopolymerization type photosensitive layers disclosed in Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J. P. KOKOKU") Nos. 54-26923 and 56-2310 and the other is photo-dimerization type photosensitive layers disclosed in J. P. KOKOKU No. 55-22781.
When the lithographic plate having the photo polymerization-type photosensitive layer is exposed to light, the silicone layer is photo-adhered to the photosensitive layer. Therefore, there is no need to add an adhesion assistant (such as a silane coupling agent) to the silicone rubber layer in order to facilitate the adhesion between the rubber layer and the photosensitive layer. In case of such a photo-adhesion, only the silicone rubber layer can be removed from the unexposed area without dissolving the photopolymerization layer, by developing the plate with a solvent capable of swelling the silicone layer. However, the reproducibility of halftone dots in the shadow area, in which the quantity of light is insufficient, is not always sufficient, because the photo-adhesion technique is essentially employed in this image-forming process. In order to solve this problem, there has been proposed a process wherein a adhesion assistant such as an aminosilane coupling agent is incorporated in the silicone rubber layer to firmly bond this layer with the photosensitive layer, and wherein a developer capable of dissolving the photosensitive layer is used to dissolve the photopolymerization layer together with the rubber layer in the unexposed area to form an image. Although the reproducibility of the halftone dots in the shadow area is improved by this process, the halftone dots in the highlight area become poor with the elapse of time, because the adhesion power between the photosensitive layer and the silicone rubber layer is gradually increased and the developability is deteriorated.
In the lithographic plate having the photodimerization-type photosensitive layer, it is necessary to incorporate an adhesion assistant such as a silane coupling agent in the silicone rubber layer in order to firmly and previously bond the silicone rubber layer with the photosensitive layer, because a sufficient photo-adhesion is not obtained unlike the lithographic plate having the photopolymerization-type photosensitive layer. The lithographic plate of this type also has a defect that the reproducibility of the halftone dots in the highlight area is deteriorated with the elapse of time.